A highly integrated semiconductor device has been desired in recent years, and thus there is a need for forming a finer pattern. In a photolithography process, in order to obtain a higher resolution for a finer pattern, it is necessary to reduce a thickness of a resist film formed on a semiconductor wafer, which is used when a pattern is formed thereon by dry-etching.
However, when a thickness of a resist film is reduced so as to form a finer pattern, a sufficient etching selectivity of a film to be etched cannot be taken relative to the resist film, which results in poor patterning.
In order to avoid this disadvantage, a double-layered resist has been conventionally used. The double-layered resist is manufactured by, for example, forming a lower layer resist film for planarization on a film to be etched, and forming, on the lower layer resist film, a photosensitive resist film as an upper layer resist film.
In the double-layered resist, the upper photosensitive resist film contains Si. A resist pattern is formed on the photosensitive resist film by exposure and development. Then, the lower layer resist film is etched (dry-developed), using the patterned upper layer resist film as a mask. Finally, the film to be etched is etched, using the upper layer resist and the lower layer resist as a mask.
In a series of these etching steps, when the lower layer resist film is etched with the upper layer resist film used as a mask, an O2-based gas mainly containing O2 has been conventionally used. Since SiO2 is generated on the upper layer resist film by the O2-based gas, the lower layer resist can be etched at a high selectivity relative to the upper layer resist. Thus, a thickness of a residual film of the upper layer resist film can be increased.
However, when a film is etched by using the O2-based gas, etching cannot be accurately performed, and thus an accurate pattern cannot be obtained. To be specific, as shown in FIG. 4, a longitudinal section of the etched part has a so-called bowing shape which degrades a controllability of a CD (Critical Dimension) shift value. As shown in FIG. 3, the term “CD shift value” herein means a value obtained by subtracting an uppermost CD value (top CD) of an etched organic material layer 64 from a lowermost CD value (bottom CD) thereof. Thus, the CD shift value takes a plus (+) value when a bottom part is wider than a top part, while it takes a minus (−) value when the bottom part is narrower than the top part.
A use of a mixed gas containing an H2 gas and an N2 gas as an etching gas has been considered. When such a mixed gas is used, etching can be accurately performed to provide a well-shaped pattern, because a bowing does not readily occur and the CD shift can be restrained.
However, a use of the mixed gas containing the H2 gas and the N2 gas lowers an etching selectivity of the lower layer resist relative to the upper layer resist. Thus, a thickness of the upper layer resist is unavoidably decreased when the lower layer resist is etched, which impairs the advantages of the double-layered resist.